A typical electrostatographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image recorded on the photoconductive member is transferred to a support material such as paper either directly or via an intermediate transport member. The developed image on the support material is generally subjected to heat and/or pressure to permanently fuse it thereto.
Two types of developer materials are typically employed in electrostatographic printing machines. One type of developer material is known as a dry developer material and comprises toner particles or carrier granules having toner particles adhering triboelectrically thereto. Another type of developer material is a liquid developer material comprising a liquid carrier or dispersant having toner particles dispersed therein.
When using a liquid developer material, excess liquid carrier such as Isopar (a non-polar decane solvent) frequently adheres to the photoconductive member and is transferred to the transport member (if any) and support material. This liquid carrier later evaporates into the air. Usually about 0.5 grams of liquid carrier is absorbed by a sheet of copy paper and carried out in each copy.
As the image is fused by heating the toner above its melting point and allowing it to flow into pores and/or surface irregularities of the support material, the heat required to fuse the image vaporizes a large percentage of the liquid carrier. It is desirable to remove the liquid carrier from the support material to minimize image show-through and to prevent problems associated with it emerging from the support material later. Additionally, increasing the solids content of an image before transferring it to an intermediate transport member greatly improves the ability of the toner particles to form a high resolution image on the transport member and thus on the support material.
The amount of liquid carrier in the tone image can be limited by a metering system. The most common metering technique is the reverse roll.
Reverse roll doctoring and corona doctoring reduce the amount of liquid carried out by the copy sheet from about 0.5 grams to about 0.12 grams per copy. Reverse roll doctoring provides superior background clean-up by having sufficient shear force to remove all the liquid carrier except the liquid carrier interstitially trapped in the toner image. However, very close spacing is required to do an effective job. It is particularly difficult to maintain this close spacing over large dimensions in applications such as color proofing master, and other graphic arts.
An air knife could also remove excess liquid carrier. However, the toner particles adhering to the latent image may also be removed, thereby disturbing the image.
Various techniques have been devised for removing excess liquid carrier from an imaging member.
U.S. Pat. No. 4,286,039 (Landa et al) discloses an image forming apparatus comprising a deformable polyurethane roller, which may be a squeegee roller or blotting roller which is biased by a potential having a sign the same as the sign of the charged toner particles in a liquid developer. The bias on the polyurethane roller is such that it prevents streaking, smearing, tailing or distortion of the developed electrostatic image and removes much of the liquid carrier of the liquid developer from the surface of the photoconductor.
U.S. Pat. No. 4,299,902 (Soma et al) discloses an image forming apparatus comprising an elastic roller which squeezes out and absorbs excess liquid developer. The elastic roller is comprised of a central roller, a porous elastic member wrapped around the roller, and an outermost elastic member with a plurality of penetrating pores.
U.S. Pat. No. 4,392,742 (Landa) discloses a cleaning system for a liquid developer electrophotographic copier comprising a roller formed with a resilient material, such as a closed-cell elastomer, having externally exposed, internally isolated surface cells. During an operation, the excess liquid on an imaging surface is absorbed by the cleaning roller. The cleaning roller is then compressed to squeeze out liquid from the roller, leaving the roller dry.
U.S. Pat. No. 4,878,090 (Lunde) discloses a development apparatus comprising a vacuum source which draws air around a shroud to remove excess liquid carrier from the development zone.
U.S. Pat. No. 4,879,197 (Kohmura et al.) discloses a pair of squeeze rollers for an electrophotographic machine comprising a metal roll with an elastomeric roller wrapped around the metal. The squeeze rollers remove excess developer from a photosensitive material.
U.S. Pat. No. 3,757,398 (Urban) discloses a squeezing roller comprised of a thin layer of synthetic material which squeezes liquid from textile webs.
U.S. Pat. No. 5,023,665 (Gundlach) discloses an excess liquid carrier removal apparatus for an electrophotographic machine. The apparatus is comprised of an electrically biased electrode having a slit therein coupled to a vacuum pump. The vacuum pump removes, through the slit in the electrode, liquid carrier from the space between the electrode and the photoconductive member. The electrical bias generates an electrical field so that the toner particle image remains undisturbed as the vacuum withdraws air and liquid carrier from the gap.
U.S. Pat. No. 4,607,947 (Ensing et al) discloses a circulating cleaning member comprising a multiplicity of spaced-apart openings or perforations. A surface of the cleaning member collects residues of toner from a surface to be cleaned.